SYSLOG(3) BSD Library Functions Manual SYSLOG(3)NAME
syslog, syslog_r, vsyslog, vsyslog_r, syslogp, syslogp_r, vsyslogp, vsyslogp_r, openlog,
openlog_r, closelog, closelog_r, setlogmask, setlogmask_r -- control system log
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <syslog.h>
void
syslog(int priority, const char *message, ...);
void
syslog_r(int priority, struct syslog_data *data, const char *message, ...);
void
syslogp(int priority, const char *msgid, const char *sdfmt, const char *message, ...);
void
syslogp_r(int priority, struct syslog_data *data, const char *msgid, const char *sdfmt,
const char *message, ...);
void
openlog(const char *ident, int logopt, int facility);
void
openlog_r(const char *ident, int logopt, int facility, struct syslog_data *data);
void
closelog(void);
void
closelog_r(struct syslog_data *data);
int
setlogmask(int maskpri);
int
setlogmask_r(int maskpri, struct syslog_data *data);
#include <stdarg.h>
void
vsyslog(int priority, const char *message, va_list args);
void
vsyslog_r(int priority, struct syslog_data *data, const char *message, va_list args);
void
vsyslogp(int priority, const char *msgid, const char *sdfmt, const char *message,
va_list args);
void
vsyslogp_r(int priority, struct syslog_data *data, const char *msgid, const char *sdfmt,
const char *message, va_list args);
DESCRIPTION
The syslog() function writes message to the system message logger. The message is then
written to the system console, log files, logged-in users, or forwarded to other machines as
appropriate (see syslogd(8)).
The message is identical to a printf(3) format string, except that '%m' is replaced by the
current error message. (As denoted by the global variable errno; see strerror(3).) A
trailing newline is added if none is present.
The syslog_r() function is a multithread-safe version of the syslog() function. It takes a
pointer to a syslog_data structure which is used to store information. This parameter must
be initialized before syslog_r() is called. The SYSLOG_DATA_INIT constant is used for this
purpose. The syslog_data structure and the SYSLOG_DATA_INIT constant are defined as:
struct syslog_data {
int log_file;
int connected;
int opened;
int log_stat;
const char *log_tag;
int log_fac;
int log_mask;
};
#define SYSLOG_DATA_INIT { \
.log_file = -1, \
.log_fac = LOG_USER, \
.log_mask = 0xff, \
}
The structure is composed of the following elements:
log_file contains the file descriptor of the file where the message is logged
connected indicates if connect has been done
opened indicates if openlog_r() has been called
log_stat status bits, set by openlog_r()
log_tag string to tag the entry with
log_fac facility code
log_mask mask of priorities to be logged
The vsyslog() function is an alternative form in which the arguments have already been cap-
tured using the variable-length argument facilities of stdarg(3).
The syslogp() variants take additional arguments which correspond to new fields in the sys-
log-protocol message format. All three arguments are evaluated as printf(3) format strings
and any of them can be NULL. This enables applications to use message IDs, structured data,
and UTF-8 encoded content in messages.
The message is tagged with priority. Priorities are encoded as a facility and a level. The
facility describes the part of the system generating the message. The level is selected
from the following ordered (high to low) list:
LOG_EMERG A panic condition. This is normally broadcast to all users.
LOG_ALERT A condition that should be corrected immediately, such as a corrupted system
database.
LOG_CRIT Critical conditions, e.g., hard device errors.
LOG_ERR Errors.
LOG_WARNING Warning messages.
LOG_NOTICE Conditions that are not error conditions, but should possibly be handled spe-
cially.
LOG_INFO Informational messages.
LOG_DEBUG Messages that contain information normally of use only when debugging a pro-
gram.
The vsyslog_r() is used the same way as vsyslog() except that it takes an additional pointer
to a syslog_data structure. It is a multithread-safe version of the vsyslog() function
described above.
The openlog() function provides for more specialized processing of the messages sent by
syslog() and vsyslog(). The parameter ident is a string that will be prepended to every
message. The logopt argument is a bit field specifying logging options, which is formed by
OR'ing one or more of the following values:
LOG_CONS If syslog() cannot pass the message to syslogd(8) it will attempt to write the
message to the console (``/dev/console'').
LOG_NDELAY Open the connection to syslogd(8) immediately. Normally the open is delayed
until the first message is logged. Useful for programs that need to manage
the order in which file descriptors are allocated.
LOG_PERROR Write the message to standard error output as well to the system log.
LOG_PID Log the process id with each message: useful for identifying instantiations of
daemons. (This PID is placed within brackets between the ident and the mes-
sage.)
The facility parameter encodes a default facility to be assigned to all messages that do not
have an explicit facility encoded:
LOG_AUTH The authorization system: login(1), su(1), getty(8), etc.
LOG_AUTHPRIV The same as LOG_AUTH, but logged to a file readable only by selected individu-
als.
LOG_CRON The cron daemon: cron(8).
LOG_DAEMON System daemons, such as routed(8), that are not provided for explicitly by
other facilities.
LOG_FTP The file transfer protocol daemon: ftpd(8).
LOG_KERN Messages generated by the kernel. These cannot be generated by any user pro-
cesses.
LOG_LPR The line printer spooling system: lpr(1), lpc(8), lpd(8), etc.
LOG_MAIL The mail system.
LOG_NEWS The network news system.
LOG_SYSLOG Messages generated internally by syslogd(8).
LOG_USER Messages generated by random user processes. This is the default facility
identifier if none is specified.
LOG_UUCP The uucp system.
LOG_LOCAL0 Reserved for local use. Similarly for LOG_LOCAL1 through LOG_LOCAL7.
The openlog_r() function is the multithread-safe version of the openlog() function. It
takes an additional pointer to a syslog_data structure. This function must be used in con-
junction with the other multithread-safe functions.
The closelog() function can be used to close the log file.
The closelog_r() does the same thing as closelog(3) but in a multithread-safe way and takes
an additional pointer to a syslog_data structure.
The setlogmask() function sets the log priority mask to maskpri and returns the previous
mask. Calls to syslog() with a priority not set in maskpri are rejected. The mask for an
individual priority pri is calculated by the macro LOG_MASK(pri); the mask for all priori-
ties up to and including toppri is given by the macro LOG_UPTO(toppri). The default allows
all priorities to be logged.
The setlogmask_r() function is the multithread-safe version of setlogmask(). It takes an
additional pointer to a syslog_data structure.
RETURN VALUES
The routines closelog(), closelog_r(), openlog(), openlog_r(), syslog(), syslog_r(),
vsyslog(), vsyslog_r(), syslogp(), syslogp_r(), vsyslogp(), and vsyslogp_r() return no
value.
The routines setlogmask() and setlogmask_r() always return the previous log mask level.
EXAMPLES
syslog(LOG_ALERT, "who: internal error 23");
openlog("ftpd", LOG_PID | LOG_NDELAY, LOG_FTP);
setlogmask(LOG_UPTO(LOG_ERR));
syslog(LOG_INFO, "Connection from host %d", CallingHost);
syslog(LOG_INFO|LOG_LOCAL2, "foobar error: %m");
syslogp(LOG_INFO|LOG_LOCAL2, NULL, NULL, "foobar error: %m");
syslogp(LOG_INFO, "ID%d", "[meta language=\"en-US\"]",
"event: %s", 42, EventDescription);
For the multithread-safe functions:
struct syslog_data sdata = SYSLOG_DATA_INIT;
syslog_r(LOG_INFO|LOG_LOCAL2, &sdata, "foobar error: %m");
SEE ALSOlogger(1), syslogd(8)
The BSD syslog Protocol, RFC, 3164, August 2001.
The syslog Protocol, Internet-Draft, draft-ietf-syslog-protocol-23, September 2007.
HISTORY
These non-multithread-safe functions appeared in 4.2BSD. The multithread-safe functions
appeared in OpenBSD 3.1 and then in NetBSD 4.0. The async-signal-safe functions appeared in
NetBSD 4.0. The syslog-protocol functions appeared in NetBSD 5.0.
CAVEATS
It is important never to pass a string with user-supplied data as a format without using
'%s'. An attacker can put format specifiers in the string to mangle your stack, leading to
a possible security hole. This holds true even if you have built the string ``by hand''
using a function like snprintf(), as the resulting string may still contain user-supplied
conversion specifiers for later interpolation by syslog().
Always be sure to use the proper secure idiom:
syslog(priority, "%s", string);
With syslogp() the caller is responsible to use the right formatting for the message fields.
A msgid must only contain up to 32 ASCII characters. A sdfmt has strict rules for paranthe-
sis and character quoting. If the msgfmt contains UTF-8 characters, then it has to start
with a Byte Order Mark.
BSD May 3, 2010 BSD